Iron Man himself would've been proud.

I get it!  You're turning 50.  You have lots of money.  You're kind of a big deal.  Why not throw yourself a huge birthday party?  You only turn 50 once.

I could totally see Downey's Iron Man character, Tony Stark, doing something like this.

He rented out an airport hangar, invited a bunch of friends, and hired Duran Duran to open up for Steely Dan  (Side note: I freaking LOVE Duran Duran).

That's right...he had his very own birthday concert with included 80's/90's icons Duran Duran.

Here's a clip of Robert Downey Jr telling Letterman all about it.